Spark plug having composite insulator



United States Patent 174/70 .1 ux 2,645,819 7/1953Peters...........................l74/l70(.l)UX

MasarniOuki 2,635,975 4/1953 Pouzet................. Nagoya; 1 A 1 Nogagztoshisuzukiokauki'shihpan Primary Examiner-Laramie E. Askin pp g 161970 Attorney-Cushman, Darby and Cushman Continuation-impart of Ser. No.730,305,

at: 13? abandoned ABSTRACT: A spark plug having a composite insulatorcom- Nlppolldem xabushmxakha prising a leg portion and a head portionformed of ceramic Kafiybshi chum! Japan materials of differentproperties and compositions. in the insuacorponuon hp; later, the legportion is formed of the first ceramic material consisting essentiallyof beryllium oxide'which has a high heat conductivity and the headportion is formed of the second ceramic material consisting essentiallyof aluminum oxide which has a high mechanical strength, these portionsbeing firmly chemically bonded to each other through at least oneintermediate layer having a composition of 1-55 percent by weight ofberyllium oxide, 89 percent by weight of alu- [72] Inventors [22] FiledPatented [73] Assignee [54] SPARK PLUG HAVING COMPOSITE INSULATOR 9Claims, 4 Drawing Figs.

minum oxide and 5-25 percent by weight of an additive, provided that, inthe compositions of the respective layers, the sum of BeO and M 0 islarger than 75 percent by weight and smaller than percent by weight. Thecompositions change gradually from said first ceramic material to saidsecond [5 6] References Cited UNITED STATES PATENTS ceramic material.Said additive generally consists of at least one member selected fromthe group consisting of MgO, SiO CaO, BaO, TiO ZrO Fe O MO and MnO andpreferably consists of about 5 25 percent by weight of two to threemembers selected from the group consisting of MgO, SK), and CaO.

313/137 313/137 l74/209X 174/209X m m m "uni n n "r. r e fi w .Dme f wuac u B818 376 0 35 9999 iii] ///l 6694 2794 3964 ,33 2038 3354 72 0 5 22SPARK PLUG HAVING COMPOSITE INSULATOR CROSS-REFERENCES TO RELATEDAPPLICATION This is a Continuation-ln-Part Application of US.application Ser. No. 730,305, filed on May 20, 1968, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to the manufacture of a composite insulator having excellentthermal characteristics for spark plugs mounted on internal combustionengines for automotive vehicles, said insulator being divided into a legportion consisting of a beryllia ceramic material having a high heatconductivity and a head-portionconsisting of an alumina ceramic matedmaterial having high mechanical strength. These two portions are notconnected with each other by any generally known cementation process ormechanical method, but are formed together as a unit with a graduallychanging composi- U)".

2. Description of the Prior Art Previously known electrical insulatorsfor spark plugs of this kind have been madeof an alumina ceramicmaterial consisting essentially of aluminum oxide and have a uniformcomposition throughout the insulator body. As is well known, thealuminum ceramic material has a high mechanical strength, but the heatconductivity is relatively low compared with that of metals. Therefore,when such an insulator having a relatively large length of thelegportion is fitted in a spark plug, a large amount of heat is appliedto the insulator and the temperature at the exposed end of the legportion tends to increase, resulting in a tendency to give rise topreignition. On the other hand, an attempt to prevent undesirablepreignition by shortening the length of the leg portion of the insulatorresults in'a defect that the insulation is deteriorated by carbon andthe like deposited during the running of the vehicles at a low speed.Especially, for mounting on a racing car engine a spark plug must have asatisfactory degree of heat dissipation, and for this purpose it hasbeen common practice to employ an insulator having a length of its legportion of about 3 to 4 millimeters. In the near future, the length ofthe leg portion will be further shortened and will possibly be reducedto zero in an extreme case. In such a case, the problem of insulationdeterioration will become more outstanding.

' In order to eliminate these vital defects, at present various kinds ofinsulators for spark plugs having varying lengths of the leg portionthereof are manufactured to suit various operating conditions. For thispurpose, it has recently been proposed to make an electrical'insulatorfor spark plugs of a ceramic material having .an excellent heatconductivity. Among ceramic materials which may be used at hightemperatures, a beryllia ceramic material consisting essentially ofberyllium oxide has the highest heat conductivity. However, the berylliaceramic material has a mechanical strength which is only about a half ofthat of the alumina ceramic material consisting essentially of aluminumoxide and is very expensive. The insulator for spark plugs must have ahigh mechanical strength at several parts thereof including the partprojecting from the outer shell of the spark plug. Furthermore, theinsulator occupies a greater part of the cost of a spark plug, andthusthe cost of the insulator is also regarded as an important factor.Therefore, a filler material having a high mechanical strength such asaluminum oxide may be added to the beryllia ceramic material to improvethe mechanical strength of the insulator and to reduce the' cost of theinsulator compared with an insulator for spark plugs made of berylliaceramic material alone.

SUMMARY OF THE lNVENTION the leg portion of the insulator, where a highheat conductivity is required, of a beryllia ceramic material consistingessentially of beryllium oxide, forming the head portion of theinsuceramic material obtained by adding asmall amount of an additive(oxide) to an appropriate mixture of the above-mentioned two ceramicmaterials. I I

It is therefore an object of the present invention to providea compositeinsulator for spark plugs having a high heat conductivity and a highmechanical strength at a low cost. Another object of the invention is toadapt a certain kind of an insulator for spark plugs to a wider range ofoperating conditions. Still another object of the invention is toprovide a composite insulator for spark plugs wherein the leg portionthereof is strongly bonded to the head portion thereof so that theinsulator may be sufficiently strong to withstand a large bending forceimparted thereto during assembly work. Still another object of theinvention is to provide a spark plug which is most suitable for use in ahigh speed engine for racing cars.

In order to bonddifferent ceramic materials, there has been known amethod of mechanical bonding of those materials, a method involving theinsertion of an appropriate refractory material between those materialsand the mechanical bonding of the materials. A method for bondingdifferent materials by a cementation process is also generally known.Also, it is known that a rubber and a synthetic resin can be chemicallybonded by the use of a special technique due to the characteristics ofthe materials, and by particularly specifying an additive or anintermediate composition. Different ceramic materials have notheretofore been completely bondedwith a gradually changing composition.We have now found that a beryllia ceramic material and a alumina ceramicmaterial can be bonded with a gradually changing composition.

It is impossible to directly bond the ceramic material A consistingessentially of beryllium oxide with the ceramic material B consistingessentially of aluminum oxide which are used in the present invention.The material A and the material B should be equal in 1. thermalexpansion coefficient, 2. melting point, 3. sintering velocity, 4.coefficient of contraction, 5. concentration gradient and 6. reactionproduct in order to strongly bond the materials with a graduallychanging composition. Even if the material A is once bonded to thematerial B, the bonded insulator may be cracked and their bonding is notso simple.

According to the present invention, the material A and the material 18are bonded by providing at least one intermediate layer between the twomaterials having intermediate characteristics with regard to 1. thermalexpansion coefficient, 2. concentration gradient, 3. sintering velocity,etc. In the intermediate layer a mixing ratio of aluminum oxide andberyllium oxide, the number of such intermediate layers provided and thekind and amount of the additive used may be varied. In this case, thematerial A and the material B cannot be strongly bonded with a graduallychanging composition if an intermediate layer consisting of a meremixture of aluminum oxide and beryllium oxide is provided. A specialcomposition must be developed in order to obtain an intermediate bondingmaterial of desirable properties.

The present invention will be illustrated by the following examples inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. l is a vertical section of asparkplug having composite insulator according to the present invention. Fl6. 2 is a vertical sectionof a moulding apparatus employed for themanufacture of an insulator for spark plugs according to the presentinvention. HO. 3 is a graphic illustration of the performance of a sparkplug having an insulator according to the present invention comparedwith the performance of a spark plug having a previously known insulatorfor spark plugs. FIG. 4 is a graphic illustration showing therelationship between the number of intermediate layers and the bendingstrength of the bonded insulator.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a compositeinsulator for spark plugs according to the present invention is composedof a leg portion la, a head portion lb and an intermediate portion 1cconnecting the leg portion In with the head portion lb. The leg portionla is formed of a ceramic material consisting essentially of berylliumoxide which has a very high head conductivity, and the head portion lbis formed of a ceramic material consisting essentially of aluminum oxidewhich has a high mechanical strength. The intennediate portion 1c isformed of a ceramic material having an intermediate composition betweenthe compositions of said two ceramic materials. At the tip of an outershell 2 of the spark plug there is formed a grounded electrode 4opposite to a central electrode 3 which has been inserted into an axialbore in the leg portion of the insulator. The leg portion is shrunk ontothe head portion of the insulator lb by well known shrinkage means suchas by electrical heating shrinkage. A rod for a high voltage terminal 5is electrically connected with the central electrode 3. Numeral 6 is asealing member. The structures of the outer shell 2, central electrode3, grounded electrode 4, rod for high voltage terminal 5 and sealingmember 6 are similar to those of previously known spark plugs.

The manufacture of a composite insulator 1 according to the presentinvention will be described with regard to a spark plug having theabove-mentioned structure.

A moulding apparatus as shown in FIG. 2 was used for the manufacture ofthe composite insulator for spark plugs. The moulding apparatuscomprised a cylindricalrubber mould 7 to which a moulding pressure wasapplied by the use of oil pressure from the outer periphery toward thecenter thereof. A core 9 had an integral needle 8 for forming an axialbore for receiving the central electrode and the rod for the highvoltage terminal. A lower mould 10 provided a moulding cavity togetherwith said cylindrical mould 7.

The ceramic materials having compositions A, B and C as shown in thefollowing Table l, which were necessary to form the desired compositeinsulator for spark plugs, were first prepared.

The composition A consisted essentially of beryllium oxide and thecomposition C consisted essentially of aluminum oxide. In case the theintermediate layer comprises only one layer as shown in Table l, thecomposition B for the intermediate a layer consists of l0-30 percent byweight of Be0, 55-85 percent by weight of Al fl and 5-15 percent byweight of an additive provided that the sum of Bell and A1 0. I

formed by the cylindrical rubber mould 7 and the lower mould 10 of themoulding apparatus as shown in FIG. 2. Ceramic material 12 having thecomposition B was then charged in a small amount. Finally, a ceramicmaterial 13 having the composition C was charged as the uppermost layer.After completing the charging of these ceramic materials into thecylindrical rubber mould 7, a suitable moulding pressure of 200 to 1,000kg/em was applied to the outer periphery of said cylindrical mould 7 toeffect compression moulding. The moulded body was then subjected tocutting or grinding into the desired shape and fired at a suitablefiring temperature of about l600 C. During the firing step, the ceramicmaterial 1 I having the composition A reacted with the ceramic material12 having the composition B at the bonded portion. A reaction product ofchrysoberyl (Be0**Al O was partly formed and these ceramic materialswere firmly bonded to each other. Thus, the leg portion of the insulatora formed of the beryllia ceramic material and the head portion of theinsulator lb formed of the alumina ceramic material were firmly bondedby the bonded portion 10 formed of the special ceramic material havingan intermediate composition between the compositions of said ceramicmaterials to form a composite insulator for spark plugs l as'shown inFIG. 1. As seen from FIG. 1, the bonded portion 10 did not show adistinct change in composition owing to the formation of said reactionproduct and a firm bonding was obtained possessing a gradually changingcomposition.

Such a composite insulator for spark plugs of such a structure showshigh mechanical strength at the upper portion including the portion,where the outer shell of the spark plug is shrunk thereon, and theportion projecting from said outer shell problems relating to mechanicalstrength. There is no problem relating to mechanical strength. Further,the leg portion of the composite insulator serving as a path of heatdissipation has a high heat conductivity and rapidly removes the largeamount of heat which is generated at the portion to effectively preventlocal overheating.

A spark plug as shown in FIG. 1 having the composite insulator for sparkplugs according to the present invention and a spark plug having a priorart composite insulator for spark plugs formed of the known aluminaceramic material consisting essentially of aluminum oxide were comparedwith regard to the relationship between their preignition limits and thelengths of the leg portions of the insulators. Referring to FIG. 3, thepreignition limit is taken as an ordinate and the length is millimetersof the leg portion of the insulator is taken as the abscissa. FIG. 3shows that the relationship between the preignition limit and the lengthof the leg portion of the insulator is represented by a straight line Bin the case of the former and by a straight line F in the case of thelatter. At the same length of the leg portion of the insulator, thepreignition limit of the former was about 1.5 times that of the latter.Also, at the same preignition limit, the length of the leg portion ofthe composite insulator of the former was about 2-3 mm. longer than thatof the latter.

A spark plug having a composite insulator which has a selected length ofthe leg portion of the insulator according to the present invention maybe advantageously used under both driving conditions in one of which avehicle travels a long distance frequently running uphill or under aheavy load, and in the other of which a vehicle travels a short distanceon a flat road in the city. Even under the former driving conditions inwhich the spark plug tends to be burnt, the very high heat conductivityof the leg portion of the insulator produces such a striking effect thata large amount of heat applied to the leg portion is quickly dissipatedthereby to substantially eliminate any undesirable temperature increaseat the lower end of the leg portion and to avoid preignition. Also,under the latter driving conditions in which the spark plug is liable tobe fouled with carbon and other deposits the considerable length of thetion of carbon and the like materials at the leg portion and toeliminate undesirable deterioration of the insulation resistance. Inother'words, a single kind of spark plug may be adapted to a wide rangeof operating conditions.

The sum of Bel) and M is larger than 75 percent by weight and smallerthan 95 percent by weight. The additive is the same as in the case ofone intermediate layer.

As shown in FIG. 4, the bending strength of the bonded in- The presentinvention is not limited to the specific embodi- 5 sulator variesaccording to the number of intermediate layers. ment illustrated above.The number of the intermediate layers If there is not present at leastone intermediate layer, the elec-i may be also two, three, four andfive. In each case, the comtrical strength and mechanical strength ofthe spark plug willi positions of the ceramic materials for forming eachportion of be insufficient for the spark plug to be put into practicaluse. the composite insulator spark plugs according to the presentRegarding the respective points plotted on a curve showingf inventionare limited as mentioned below. lo the relationship between the bendingstrength of the bonded In case of two intermediate layers, thecompositions of the insulator and the number of intermediate layers insaid FIG. 4, respective layers are shown in Table 2. the representativeexamples of the compositions of the respective layers are shown asfollows: TABLE 2 The first ceramic layer consists of 95.0 percent byweight of l Be0, I.0 percent by weight of M 0, and 4.0 percent by weightComwmon of an additive. The second ceramic material consists of 95.0

percent by weight of A1 0; and 5.0 percent by weight of an ad-Component, percent by Weigh ?;2 mg ditive. The compositions ofintermediate layers are shown below for each number of the intermediatelayers. The inter- 90-98 25-40 10-25 2 mediate layers are successivelynumbered from the side {:3 gig 33g closest to the first ceramic layer.

The sum of Bell and A50; is larger than 80 percent by weight 25 andsmaller than 95 percent by weight. The additive is the same as in thecase of one intermediate layer.

In case of three intermediate layers, the compositions of the respectivelayers are shown in Table 3.

TABLE 3 Composition Component, percent by wt. First First Second ThirdSecond ceramic layer layer layer ceramic B00 00-45 -00 5-15 Ami"-.. 1-540-65 55-80 60-65 90-06 Additive 1-0 5-15 5-15 10-25 2-10 The sum ofBell and ALO, is larger than 75 percent by weight and smaller than 95percent by weight. The additive is the same as in the case of oneintermediate layer.

In case of four intermediate layers, the compositions of the respectivelayers are shown in Table 4.

TABLE 4 Composition First First Second Third Fourth Second Component,percent by wt. ceramic layer layer layer layer ceramic B00 90-08 35-50-65 10-20 3-10 Alloi 1-5 -60 50-75 60-80 65-87 00-03 Additive 1-0 5-15 75-15 10-20 10-25 2-10 The sum of Be0 and 111.0, 8 larger than 75 percentby weight and smaller than 95 percent by weight. The additive is thesame as in the case of one intermediate layer.

In the case of live intermediate layers, the compositions of therespective layers are shown in Table 5.

TABLE 5 Composition First First Second Third Fourth Filth SecondComponent, percent by wt. ceramic layer layer layer layer layer ceramic(1) In the case of one intermediate layer:

First layer Percent by wt.:

(2) In the case of two intermediate layers:

First layer Second layer 27. 12. 0 63. 0 73. 0 g 0. 5. 5 S 5. 0 5. 0 CaO4. 5 4. 5

Norm-Bending strength of bonded insulator: 17.8 kgJmm. (3) In the caseof three intermediate layers:

First Second Third layer layer layer N01E.Bending strength of bondedinsulator: 20.1 kgJmm.

(4) In the case of four intermediate layers:

First Second Third Fourth layer layer layer layer Nora-Bending strengthof bonded insulator: 19.9 kgJmm.

(5) In the case of five intermediate layers:

No'rE.-Bending strength of bonded insulator: 20.2 kg/mrn.

As described above, according to the prCLLnt invention, the leg portionof a composite insulator is formed of a beryllia ceramicmaterialconsisting essentially of beryllium oxide, and the head portionof the composite insulator is formed of an alumina ceramic materialconsisting essentially of aluminum oxideqThereby, a composite insulatorfor spark plugs of a high heat conductivity and a high mechanicalstrength may be provided which is excellent in heat conductivity at itsportion requiring a high heat conductivity and it is also excellent inmechanical strength at theportion requiring a high mechanical strength.Furthermore, an effect that a spark plug can be used under a wide rangeofoperating conditions is remarkable achieved because the leg portion ofthe composite insulator is formed'of a beryllia ceramic material asdescribed above. The composite insulator for spark plugs according tothe present invention can be produced at a relatively low cost in spiteof the fact that the leg portion isformed of an expensive berylliaceramic material. becausethe leg portion occupies only about one-thirdor less of the total volume of the insulator. As the leg portion and thehead portion are bonded to each other through a ceramic material havinga particular intermediate composition between the composition of theberyllia ceramic material and the composition of'the alu'mina ceramicmaterial, the bond between the leg p'oition'andth'e head portionprovided by a reaction between thesecraniic materialsat the bondedportion of the layers is astrbnger than the boh obtained by merelybonding the leg portionto the head p tion with glass or a heat resistantcement arty mechanical fabrication. The insulator can sufficientlywithst'anda strong bending force applied thereto during theassemblyoperation.

We claim:

1. A spark plug having a composite insulator comprising a leg portionconsisting essentially of beryllium oxide and a head portion consistingessentially of aluminum oxide characterized in that said two portionsare chemically bonded to each other through at least one intermediatelayer consisting of 30 to 89 1 percent by weight of aluminum oxide, l to55 percent by weight of beryllium oxide and 5 to 25 percent by weight ofa ceramic oxide additive, provided that the sum of Be0 and Al 0 islarger than 75 percent by weight and smaller than 95 percent by weight,and that the compositions of the respective intermediate layersgradually change from that of the head portion to that of the. legportion..-

2. A spark plug according to claim 1, wherein the additive consists ofat least one member selected from the group consisting of Mg0, Si0 Ca0,Ba0,-'Ti0 Zr0 Fe 0 M0 and MnO. 3. A spark plug according to. claim 1,wherein the additive consists of about 5 to 25 percent by weight of twoto three group consisting of Mg0, Si0 and members selected from the Ca0.

4. A spark plug according to claim 1, wherein the number of saidintermediate layers is 3 to 5.

5. A spark plug according to claim 1, which on has one intermediatelayer consisting of 10 to 30 percent by weight of Be0, 55 to 85 percentby weight of A1 0 and 5 to 15 percent by weight of the additive,provided that the sum of Be0 and A1 0 is larger than 85 percent byweight and smaller than 95 percent by weight.

6. A spark plug according to claim 1, which has two intermediate layers,the composition of the first layer being 25 to 40 percent by weight ofBe0, 45 to 70 percent by weight of M 0 and 5 to 15 percent by weight ofthe additive and the composition of the second layer being 10 to 25percent by weight of Bet) and 55 to percent by weight of A1 0 and 10 to20 percent by weight of the additive, provided that the sum of Bet) andA1 0 is larger than 80 percent by weight and smaller than 95 percent byweight.

7. A spark plug according to claim 1, which has three intermediatelayers, the composition of the first layer being 30 to l third layerbeing 5 to 15 percent by weight of Be0, 60 to .percent by weight of A1 0and 10 to 25 percent by weight of the additive, provided that the sum ofB60 and Al 0 is'larger than 75 percentby weight and smaller than percentby weight. v

8. A spark plug according to claim 1, which has fourintermediate layers,the composition of the first layer being 35 to 50 percent by weight ofBot), 35 to 60 percent by weight of Al 0 and 5 to 15 percent by weightof the additive, the composition of the second layer being 20 to 35percent by weight of Be0, 50 to 75 percent by weight of M 0 and 5 to 15percent by weight of the additive, the composition of the third layerbeing 10 to 20 percent by weight of Be0, 60 to 80 percent by weight of M0 and 10 to 20 percent by weight of the additive and the composition ofthe fourth layer being 3 to l0 percent by weight of Be0, 65 to 87percent by weight of Al 0 and 10 to 25 percent by weight of theadditive, provided that the sum of Be and M 0 is larger than 75 percentby weight and smaller than 95 percent by weight.

9. A spark plug according to claim 1, which has five intermediatelayers, the composition of the first layer being 40 to 55 percent byweight of 81:0, 30 to 55 percent by weight of A50 and S to 15 percent byweight of the additive, the composition of the second layer being 25 to40 percent by weight of Be0, 45 to 70 percent by weight of A1 0 and 5 topercent by weight of the additive, the composition of the third layerbeing 15 to 25 percent by weight of Bel), 55 to 75 perlarger than 75percent by weight and smaller than percent by weight.

